1. Field of the Invention
The present invention relates to real time, curved surface imaging systems. More particularly, the present invention is directed to a laser-based, rear projector imaging system.
2. Description of the Related Art
Spherical dome imaging systems have long been used in a variety of applications ranging from planetariums and commercial entertainment systems to military flight simulators. Domed imaging visual systems are capable of generating scenes with a high degree of realism because the scene is presented to the view in many directions at once, not just from a single flat plane as in most conventional imaging systems. The capacity for producing a wide field of view is highly desirable in both commercial and military imaging systems applications.
Most conventional dome display systems in use are front projection systems. Like conventional movie theaters, front projection systems employ one or more projectors positioned on the same side of the screen as the viewer and serve to project still and/or moving images onto the screen. On conventional front projection spherical dome displays, it is often necessary to use elaborate mounting mechanisms and costly optical lens arrangements to insure a collect viewing perspective, without interfering with the projected light path. Subtle protrusions of the "rib" structure located on the exterior of the dome are frequently apparent on the interior screen surface. Because the projector and projected light rays are on the same side of the screen as the viewer, physical limitations are imposed which restrict placement and movement of the viewer. When employed in simulation and training systems, the necessity for avoiding occluding projected light rays places severe limitations on training device location and configuration. In domed imaging systems which employ multiple front projectors, the physical limitations may also undesirably restrict the number of projectors and, as a result, the number of images that the system can employ.
Conventional front-projected dome surfaces are often painted with a display screen medium to enhance visibility. This can result in irregularities being visible on the display surface, such as mottling. The viewing dome surface also must have very critical optical characteristics necessary to produce sufficient off-axis luminance. Producing dome screens with these desired optical characteristics is an expensive process requiring specialized talent to produce the required finish.
Another factor which contributes to the complexity and cost of front projection domed imaging systems is the distortions caused by off-axis projection of images onto the curved dome surfaces. Correcting such distortions requires special video projectors capable of performing such corrections. This can increase the cost of the system and require a significant amount of time.
In an effort to overcome the problems associated with front projection systems, some display systems use rear-projection techniques, often referred to as "dome displays". In actuality, such systems are not true spherical dome displays. Rather, they usually employ multi-faceted, flat panel displays, configured with multiple flat panel display screens, specifically designed to encapsulate the viewer. While such systems may provide certain advantages achieved by a true dome display, they lack a key feature of a true spherical dome display. Namely, they do not have true spherical geometry. This can result in undesirable visual irregularities or artifacts when the imaging system is employed within the exacting requirements of military flight simulation or other visually demanding applications. One such artifact is variable eye relief distance. In a true spherical dome display wherein the design eye point is at the center of the sphere, the ideal eye relief distance is equal to all points on the display surface. This is not true when using a flat panel, multi-faceted, dome display. In such flat panel displays, as the viewer's gaze moves away from the line of sight between the design eye point and the screen normal, the eye relief will increase at a rate inversely proportional to the cosine of the angle traversed. In comparison, in a true spherical dome display, the design eye relief remains constant regardless of gaze direction. As can be easily understood, an advantage of a true spherical dome over a multi-faceted, flat panel is the fixed eye relief to all points on the display surface.
When employing conventionally illuminated projectors, i.e. light valve, CRT, LCD and DMD projectors with standard lenses, the projected image size is a function of the projection lens and the throw distance. After the image size has been established, a mechanical focus adjustment procedure is employed to correct for center and off-axis corner focus of the displayed image. The available range of adjustment for off-axis focus depth is very limited. With conventional non-laser light sources, any change of screen position in relation to the position of the projector lens requires re-focusing of the display image. The focus adjustment is within the acceptable range at only a very specific distance between the projector and the display screen. If there is even a slight change in projector throw distance, it is necessary to again perform the focus adjustment procedure. Thus, when employing non-laser projectors, it is usually not possible to focus a display image on a curved dome surface without employing costly special optics to compensate for the curvature of the dome surface.
It is clear that there exists a need in the art for a full field-of-view (FOV), spherical dome display system employing a rear-projection imaging source capable of displaying images in real time on a curved surface of the dome. Such a display system should overcome the problems associated with conventional illumination light sources while being cost efficient. As will become apparent, the present invention meets all of these requirements in a unique dome display system.